Chile's Atacama Desert is the driest place on Earth. In regions closer to the Pacific coast, a sprinkling of bacteria and lichen manages to scratch out an existence, surviving on moisture from salt fogs that occasionally move in from the ocean.

But no life can survive in the parched interior, which receives only a few millimeters of rain per decade.

"There's actually quite a lot of variation in the desert. There are regions in which life is very scarce. But very nearby, you can find some organisms surviving," says David Wettergreen, a research scientist at the Robotics Institute at Carnegie Mellon University in Pittsburgh, Pennsylvania.

Wettergreen heads the Life in the Atacama project, the research group that designed and built Zoe.

Zoe will attempt to distinguish between regions that are inhabited and those that are not.

"We'll be in two field sites, one that's a little bit closer to the coast, and we know to have fog events, and one site that is farther to the interior, that is going to be more dry," Wettergreen says.

A handful of researchers will follow along behind the rover, repeating its experiments manually to assess the accuracy of the rover's findings. If Zoe succeeds at mapping life's boundaries, its technology may be adapted for use in future missions to Mars.

Like Spirit and Opportunity, Zoe's payload includes stereo panoramic cameras for surveying and documenting its surroundings.

All three rovers also incorporate an infrared spectrometer that can gather basic information about the mineral composition of sand and rocks.

But unlike Spirit and Opportunity, Zoe also contains a fluorescence imager, designed to detect the telltale chemistry of life.

To understand how the fluorescence imager works, think black light. You know that invisible stamp you get on the back of your hand when you go to a dance club?

The ink on the stamp glows when the bouncer at the door waves a black light over it. The black light emits invisible ultraviolet (UV) light; the ink absorbs this light and radiates light back at a different frequency, which is visible to the bouncer.

That's fluorescence. The bouncer's eyes are functioning as a fluorescence detector, detecting hand-stamp ink.

Zoe's fluorescence imager works in a similar way. Instead of hand-stamp ink, though, it is designed to detect chlorophyll, the biological molecule involved in photosynthesis. Chlorophyll is naturally fluorescent.

Mounted beneath the rover's chassis are red, blue and green LEDs. Attached to the fluorescence imager is a set of filters; each filter allows light of a different color to pass through it.

Zoe flashes one of its LEDs, then the fluorescence imager captures an image through one of its filters. The rover performs this operation several times, using a different LED-filter combination each time.

Wherever chlorophyll is present in the material being photographed, it will show up as a bright spot in certain images.

Because "at the surface, most organisms are going to be photosynthesizing organisms, by looking for chlorophyll, we're doing a good approximation to the search for life," Wettergreen says.

Wettergreen's team is working closely with another research group headed by Alan Waggoner, also at Carnegie Mellon, to expand the range of biological molecules Zoe can detect. Waggoner's team has developed a set of fluorescent dyes, each of which bonds to a different biological molecule.

There are dyes for DNA, protein, lipids (found in the walls of living cells) and carbohydrates. These dyes are sprayed on a target, which Zoe then images using many different LED-filter combinations.

As with chlorophyll, bright spots in certain images indicate the presence of various molecules.

"You can find DNA molecules, and amino acids, and that kind of thing scattered almost anywhere you look, but when you have all those different molecules of life together in a single pixel in your image, that's probably indicating that there's something alive there," says Wettergreen.

This is the second of three field seasons for the Life in the Atacama project. In the first field season, in April 2003, researchers used a predecessor to Zoe named Hyperion.

Work the first year focused on testing out individual scientific instruments and the software that allowed Hyperion to operate autonomously over long traverses across the desert.

This year, some of the scientific instruments will be more tightly integrated into the rover, and work will continue to develop its auto-navigation software.

Because the goal of the project is to map the distribution of life over a wide area the rover used needs to be able to travel for many kilometers without human intervention.

"We believe that by looking in more places we increase our chances of finding evidence for life and building up that map of the distribution," Wettergreen says.

"Because we're operating under this hypothesis that life is going to vary by micro-habitats in the desert - it might exist on one rock but not another rock, or it might exist in an area where fog is - our hypothesis is that we have to look a lot of places, we have to examine a lot of sites. So we're operating in quick survey mode."

A future mission to survey a region of Mars for signs of life may employ a similar approach.

Zoe's software, like Hyperion's, takes important strides in this direction: the rover is able to alter its route to avoid obstacles, align itself so that its solar panels receive maximum sunlight, and strike off on a side trail to investigate an interesting object that it was not instructed to examine.

In 2003, Hyperion traveled a total of 10 kilometers (6.2 miles) on autopilot, over a period of 5 days. In once case, the rover traversed more than 1 kilometer (0.6 miles) on a single command from its operators.

Plans for the second field season call for Zoe to travel 5 times as far as Hyperion did (50 km, or 31.1 miles), over a period of 10 days. In its final field season, in the latter half of 2005, Wettergreen hopes to send a rover (either Zoe or its successor) across the Atacama on a journey of more than 180 kilometers (112 miles) over the course of 10 days.

By comparison, Spirit, which holds the distance record for martian rovers, took about 2 months to travel 2 kilometers (1.2 miles).

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